Along with advances in applications of lithium ion secondary batteries in electric cars and hybrid electric cars, it is required to attain a higher energy density. Currently, LiFePO4 and the like are put to practical use as a positive electrode material having high safety, but the average operation potential thereof is low, i.e., 3.4 V based on Li. A positive electrode active material having a higher operation potential is essential to attain a higher energy density of a lithium ion secondary battery.
Recently, Li(Fe, Mn)SO4F of a Tavorite type or triplite type crystal structure has been widely studied as a positive electrode material capable of meeting the requirement described above. The Tavorite type Li(Fe, Mn)SO4F has an average operation potential of 3.6 V (vs. Li/Li+) and the triplite type Li (Fe, Mn)SO4F has an average operation potential of 3.9 V (vs. Li/Li+), and thus are promising as a next-generation secondary battery positive electrode material having a high energy density. These positive electrode materials, however, have a high deliquescence, and thus have a defect in which their properties are degraded due to moisture in the air in production of a battery, or moisture generated inside a battery.